Abstract: Hydraulic Residence Time Distributions (RTDs) in an inner loop threephase biological fluidized bed reactor were measured under different flow regimes using pulse stimulusresponse technique method. The number of tankinseries measured was approximately 1 in aerobic area of the reactor, therefore the reactor can be seen as a Continued Stirred Tank Reactor (CSTR). By comparing different Hydraulic Retention Time (HRT), the authors found that at HRT of 1.5 h, the capacity usage reached the optimum level. Meanwhile the maximum NH4-N removal efficiency, more than 90%, was also obtained when HRT was 1.5 h The results validates the reliability of RTD test and provides the basis of sizing and scaling up of the reactor.

Key words: threephase biological fluidized bed, RTD, NH₄N removal